This is mere supposition on my part, and not a definitive answer. The real answer would need to come from either the designer or someone connected to them, I guess. I have contacted people around the designer and invited them to comment/answer, but that appears to still be "pending"...
In response, or - rather - with respect to, to the comments below by tjb1 and Tom, I have been reading around the topic, and found some interesting articles.
The excerpt below from "An exploration of the topic", backs up Tom's argument about constraining the rods at both ends being bad practice, and explains why it is so. Whilst acknowledging that a loose, or floating end, is still an issue that needs to be dealt with, the article also states that constraining the end is not really a solution, as it can create more problems elsewhere (I have added the bold highlighting to the relevant text):
Wobble is pretty simple. Because the lead screw is mounted rigidly to
the motor, it needs to be perfectly straight and square to the axis
its trying to move. If it's not, as the motor rotates, that offset will
be converted into an elliptical motion instead of turning in a perfect
circle. In big CNC world, as the axis is normally bolted to a
huge/heavy table which refuses to yield to this movement, it results
in breaking your motor or motor coupling (weakest point in the
connection). This is what flex couplings are designed to fix – if you
can’t guarantee a perfectly square mating between motor and axis, you
use a coupler with flex so that any movement can be soaked up in
flexing the coupler and the lead screw/rod moves in a nice circle
without busting your motor or mounts. This is true when you have a
good solid supported connection on the lead screw like you’d see on a
‘proper’ CNC. However on the Solidoodle, as the top end of the Z-Axis
is ‘unsupported’ and its only connection to anything is to the table
via a tiny little nut, its free to ‘flop about in the breeze’ so to
speak. Even a small 0.1mm offset from center can result in a much much
greater ‘wiggle’ at the top of the screw clearly visible to the eye.
Throw in even a 0.05mm bend in the rod, and it gets further amplified.
Adding better support to the axis, through a taller nut, multiple
nuts, or supporting the end, would reduce the influence of this
wobble. You do however, risk moving it to something else entirely –
for example, making the lead screw rigid would mean that the forces
would end up moving the motor itself, potentially causing fatigue
issues with how its bolted to the sheet metal case (mine already moves
a fair bit and its ‘stock’... making the screw rigid would see the
motor having to absorb all that movement instead of half of it
disappearing in movement of the screw...). Solving the motor movement
by securing it ‘better’ to the case would mean that the movement now
gets soaked up in the motor shaft and bearing, leading to premature
stepper motor death.
Thus, I had originally presumed, when first posting this question, the earlier versions of the model would be superior, as they secured the top of the z-axis screw mechanism, that would reduce the amount of "flapping about" of the loose end, which in turn would result in less wobble. However, I had not bargained for the negative consequences.
So, is the reason that the top brackets of the z-axis, in the version 4 of the frame, lost their threaded rod/leadscrew top-end constraint, therefore to prevent premature motor wear, at the expense of exhibited wobble?
